A gas turbine includes a compressor, a combustor, and a turbine. The compressor compresses air taken in from an air inlet, and generates high-temperature and high-pressure compressed air. The combustor supplies fuel to the compressed air to burn the fuel, and generates high-temperature and high-pressure combustion gas. In the turbine, a plurality of turbine nozzles and a plurality of turbine rotor blades are alternately arranged in a casing. The turbine rotor blades are driven by the combustion gas supplied to an exhaust passage, to rotate, for example, a rotor connected to a power generator. The combustion gas that has driven the turbine is released into the atmosphere after dynamic pressure thereof is converted to static pressure by a diffuser.
In a gas turbine thus configured, combustion gas acting on the turbine rotor blades is high-temperature gas. Therefore, compressed air is taken from the compressor to outside, the air is cooled by an external cooler to generate cooling air, and the turbine rotor blades are cooled by supplying the cooling air thereto.
When the cooling air is supplied from the external cooler to the turbine rotor blades, a cooling passage is provided. For example, in a cooling passage that introduces cooling air from a downstream side of the rotor to turbine rotor blades at the last stage, there can be considered a configuration such that a cooling passage extends along a rotation shaft of a rotor to a center of a disk of the turbine rotor blades at the last stage, and then extends radially outward to lead to the turbine rotor blades at the last stage. However, in this configuration, because the cooling passage extends long from the center of the disk to the turbine rotor blades at the last stage and radially outward, the strength of the disk decreases, which is not desired.
Therefore, in order not to decrease the strength of the disk, in a cooling passage shown in FIG. 6, a first passage 51 extending radially outward from the center of the disk is opened and formed in a cavity 53 provided in a annular pattern in an outer circumference of a disk 35, and a second passage 52 leading to turbine rotor blades 33a at the last stage (hereinafter, “the last-stage turbine rotor blades 33a”) and open to the cavity 53 is formed in the disk 35 that fixes the last-stage turbine rotor blades 33a. A cylindrical cooling passage cover 55 that covers the cavity 53 so as to connect respective passages 51 and 52 is provided in the outer circumference of the disk 35. In this configuration, a cooling passage 5 is divided into the first passage 51 and the second passage 52, and respective passages are formed short in the radial direction. Therefore, a decrease in the strength of the disk 35 can be prevented.
Meanwhile, when the cooling passage 5 is formed as shown in FIG. 6, a temperature difference between an upstream side (a front side) and a downstream side (a rear side) of a flow of combustion gas in the turbine centering on the cavity 53 is large, and thus distortion occurs in the cavity 53 in an axial direction of the turbine. Further, because opposite ends of a rotor 4 are supported by bearings and a central part of the rotor 4 deforms in a radial direction of the turbine due to a centrifugal force, an upstream side and a downstream side of the cavity 53 present in the outer circumference of the disk 35 constituting the rotor 4 deform so as to approach or be separated from each other in the axial direction of the turbine. Therefore, a function of absorbing the distortion due to the temperature difference and deformation due to the centrifugal force needs to be provided to the cooling passage cover 55.
Conventionally, to absorb an extension amount due to thermal deformation, there has been known a gas turbine provided with a sealing member in a sliding portion in an axial direction of a turbine (see, for example, Patent Document 1). As shown in FIG. 6, it can be assumed that the cooling passage cover 55 is divided into an upstream side and a downstream side in the axial direction of the turbine and a sealing member 551 is provided to allow a sliding movement in the axial direction of the turbine between the upstream side and the downstream side.    Patent Document 1: Japanese Patent Application Laid-open No. H11-229804